man <command> pulls up the manual page for a command.
apropos and man -k do the same thing: search through the entire text of all manual pages for a given keyword.
whatis and man -f search for just the name.
whereis shows you where the actual program, its source files, and the manual pages for it are saved on the local system.
info <command> is another, less commonly used command for pulling up manual pages for commands.
help <command> is used to show manual pages for commands that aren’t actually programs and are built into the shell.
<commandname> --help will usually get you some kind of help text. -h sometimes works as well, but that flag is often assigned to something else (like human-readable in df -h, ls -h, and free -h).